Heat shield having strain compliant matrix and method of forming same

ABSTRACT

A heat shield having a honeycomb core. The honeycomb core has a plurality of intersecting wall portions forming a plurality of cells. A strain compliant material is applied to the wall portions of the honeycomb core prior to filling of the cells with an ablative material. An ablative material at least substantially fills the cells of the honeycomb core.

FIELD

The present disclosure relates to heat shields, and more particularly to a heat shield incorporating a strain compliant matrix that is used to coat a component of the heat shield to reduce the development of stress cracks in an ablative material of the heat shield during a curing operation.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Spacecraft require safe, lightweight, affordable heat shields for protecting the spacecraft and its occupants during re-entry of the spacecraft into the Earth's atmosphere. Traditionally, the weight of the heat shield has been an important factor. The higher the weight of the heat shield panels used the lower the payload that the spacecraft will be able to carry. Previously manufactured heat shields have typically been made from mixtures of epoxy-phenolic resins with fillers, from phenolic resins with fillers, from carbon-carbon composites with backside insulation, from quartz-phenolic composites with backside insulation, or from Phenolic Infiltrated Carbon Ablator (PICA) material. However, existing solutions can often add significant weight to a spacecraft.

The cost of manufacturing previously developed heat shields for a spacecraft has also been an important concern for designers. Traditionally, the high cost of manufacturing heat shields for spacecraft has contributed significantly to the overall cost of manufacture for a spacecraft. The manufacturing of previously developed heat shields has also often required complex manufacturing processes.

When manufacturing heat shields using a curable ablative material, in some limited instances there may be a tendency for the ablative material to crack during the cure process. This can be due to shrinkage of the ablative material during a curing process. It may also be due to coefficient of thermal expansion differences between the ablative material and other substrate materials that help to form the heat shield. In the event any small or large cracks develop, then such cracks would need to be repaired subsequent to completion of the curing process and any following cool-down period.

Options for attempting to prevent cracks from developing could involve making relief cuts in one of more layers of material of the heat shield. This may reduce or eliminate the possibility of one or more large cracks developing in the ablative material during the curing process. However, such an approach would still require post-processing work to fill and repair the intentionally made relief cuts.

Another option for attempting to prevent cracks in the ablative material during the cure process would be to add chemicals to the ablative material that would attempt to limit shrinkage of the ablative material during the cure process. Obviously, this requires modification of the chemical structure of the ablative material, and such may not be desirable because of the risk that the additional chemicals might interfere with the overall effectiveness of the ablative material in acting as a heat shield.

Still another option might be to cure the ablative material of the heat shield to a less mature state. However, this option might not enable the ablative material to provide the same level of ablative performance. This option may also introduce potential problems relating to “heat soak” which may occur in situ. By “heat soak” it is meant the thermal exposure that the material would encounter during flight or mission conditions. During a normal mission of a spacecraft, each component of the spacecraft requiring a heat shield experiences different thermal environments. With the less mature cure option, if the thermal conditioning experienced by the heat shield during a mission prior to the point at which the ablative heat shield needs to function is sufficient to advance the cure of the ablative material, then it may potentially cause the same cracking that the less mature state was used to avoid. As such, this option makes the expected thermal environment that the components are expected to experience prior to the point at which ablation is required a more important consideration.

Still another option might be to cure the ablative material separately from the other components of the heat shield to which the ablative material will ultimately be secured, and then perform a secondary bonding operation to permanently attach the ablative material layer to one of the other material layers of the heat shield. However, secondary bonding would significantly increase the overall complexity of the heat shield by requiring gaps, joints and surface machining of the bond surface on the ablative material.

SUMMARY

In one aspect the present disclosure relates to a heat shield. The heat shield may comprise: a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; a strain compliant material applied to the wall portions of the honeycomb core prior to filling of the cells with an ablative material; and an ablative material that at least substantially fills the cells of the honeycomb core.

In another aspect the present disclosure relates to a method for forming a heat shield. The method may comprise: providing a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; applying a strain compliant material to the wall portions prior to filling of the cells with an ablative material; and filling the cells with an ablative material that at least substantially fills the cells of the honeycomb core.

In another aspect the present disclosure relates to a method for forming a heat shield. The method may comprise: providing a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; flowing a viscous strain compliant material over the intersecting wall portions prior to filling of the cells with a phenolic ablative material; at least substantially filling the cells of the honeycomb core with an ablative material; and curing the honeycomb core having said cells at least substantially filled with said phenolic ablative material in an autoclave.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a side view of one exemplary spacecraft making use of a heat shield formed in accordance with the teachings of the present disclosure;

FIG. 2 is a perspective view of one section of the heat shield shown in FIG. 1;

FIG. 3 is a cross section of the heat shield in accordance with section line 3-3 in FIG. 2;

FIG. 4 shows the carrier panel side of a honeycomb panel that has had venting slots cut into the cell walls;

FIG. 5 is a flowchart of exemplary operations that may be performed to make the ablative material that is used in constructing the heat shield of FIG. 1;

FIG. 6 is an illustration of a mold tool being filled with the ablative material;

FIG. 7 shows a rubber caul sheet being placed over the BPA mix and a vacuum bag being secured over the filled mold tool of FIG. 5;

FIG. 8 shows the ablative material being debulked prior to being frozen;

FIG. 9 shows the vacuum bagging material being removed from the mold tool and the frozen preform;

FIGS. 10A and 10B show perspective views of the resulting ablative preform ready to be placed into the freezer for storage or to be pressed into a honeycomb core;

FIG. 11 is a flowchart illustrating exemplary operations in forming the heat shield of the present disclosure;

FIG. 12 is a partial side cross sectional view of the ablative preform positioned over the honeycomb core, and with the preform/honeycomb core assembly positioned within a mold tool that is enclosed within a vacuum bag ready for placing into the autoclave;

FIG. 13 is an exemplary graph of the pressure and heat profiles used during green state curing of the assembly shown in FIG. 12;

FIG. 14 is an exemplary graph of the pressure and heat profiles used during post curing of the assembly shown in FIG. 12;

FIG. 15 is a perspective view of an exemplary closeout component that may be secured to a perimeter edge of the heat shield to close it; and

FIG. 16 is a simplified side view showing how a monolithic heat shield may be formed using a plurality of sections of the heat shield described in the present disclosure.

FIGS. 17A-17E illustrate a sequence of operations showing the overall approach for a monolithic heat shield that is formed by first attaching the honeycomb core to a spacecraft structure and then processing the assembly;

FIGS. 18A-18F illustrate a sequence of operations for an alternate approach for constructing a monolithic heat shield that is formed by filling the honeycomb core on a tool that matches the heat shield structure, then processing, machining and attaching the ablative panel to the spacecraft in one piece; and

FIG. 19 illustrates the honeycomb core being coated with a strain compliant material prior to the frozen preform being applied to the honeycomb core.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

Referring to FIG. 1, an exemplary spacecraft 10 is shown incorporating a heat shield 12 in accordance with one embodiment of the present disclosure. The heat shield 12 protects the spacecraft 10 and its occupants from the heat generated during reentry into the Earth's atmosphere, or during planetary entry. While the heat shield 12 is shown on a manned spacecraft, it will be appreciated that the heat shield 12 is well suited for use on a wide variety of other manned and unmanned space vehicles that are expected to encounter high temperatures on their exterior surfaces during travel through the Earth's, or a planetary atmosphere. The heat shield 12 is also potentially usable on other forms of vehicles, and possibly even on fixed (i.e., non-mobile) structures. The heat shield 12 may find use on virtually any form of mobile airborne platform or ground based vehicle, or possibly even on marine vehicles.

Referring to FIGS. 2 and 3, the heat shield 12 is shown in greater detail. The heat shield 12 includes a core 14, which in this example is a honeycomb core. For convenience the core 14 will be referred to throughout the following discussion as the “honeycomb core 14”. The honeycomb core 14 has a plurality of intersecting wall portions 15 that form a plurality of cells 15 a. An ablative material 16 is press fit into the cells 15 a of the honeycomb core 14. In FIG. 3, the honeycomb core 14 may be secured via an adhesive layer 18 to a carrier structure 20.

The honeycomb core 14 may be formed from a fabric of well known fiberglass, for example Style 120 (E-glass), which is impregnated with a phenolic resin. The honeycomb core 14 may also be formed from a Leno weave fiberglass or carbon fiber fabric having an open weave construction. This enables the ablative material 16, when compressed into the cells 15a of the honeycomb core 14, to fill the cells 15 a and become an integral portion of the wall structure of the honeycomb core 14. Prior to filling the cells 15a of the honeycomb core 14 with the ablative material 16, the honeycomb core 14 may be cleaned with a radio frequency (RF) generated plasma field so that its surfaces are thoroughly conditioned for the remaining manufacturing operations to which the honeycomb core 14 will be subjected. The plasma field cleaning treatment is a process that is commercially available. One such company performing this process is 4^(th) State, Inc., of Belmont, CA.

Referring to FIG. 4, following cleaning, and prior to filling the cells 15a with the ablative material 16, the walls 15 of the honeycomb core 14 are partially slotted, preferably using a diamond edged cutting tool, on the side of the honeycomb core 14 that will be bonded to the heat shield carrier structure 20. In FIG. 4 these slots are identified by reference numeral 21 and shown in detail on a piece of the honeycomb core 14. The slots 21 provide escape paths for ambient air that might otherwise create back pressure in the cells 15a during the subsequent honeycomb core 14 filling process, and for water and gases that evolve during a subsequently performed autoclave curing process. The air, water and gases are drawn off by a vacuum that is applied to a vacuum bag enclosing a preform that forms the ablative material 16, the honeycomb core 14, the carrier structure 20 and the tool. This process will be described in greater detail in the following paragraphs.

The carrier structure 20 may be formed as a multilayer structure from one or more metal sheets, or possibly even as a honeycomb structure having metal, for example titanium, face sheets. For convenience the carrier structure 20 has been drawn as a single metal layer in FIG. 3. The adhesive layer 18 may be formed by any suitable adhesive, but in one example HT-424 adhesive, which is an epoxy-phenolic structural film adhesive commercially available from Cytec Industries, Inc. of West Paterson, N.J., is used as the adhesive.

The ablative material 16 is uniquely formulated to form a lightweight, medium density, syntactic foam ablator compound. The ablative material 16 may comprise a mixture of phenolic resin, carbon fibers, silica (SiO₂ or Manville “Q”) fibers, phenolic microspheres and silica microspheres. The silica fibers and carbon fibers are used to provide structural reinforcement to the ablative material 16 and to enhance the thermal and ablative performance. The silica microspheres and phenolic microspheres are used as density reducing fillers that also enhance the thermal and ablative performance of the ablative material 16. The phenolic resin, in one example, may be Plenco 11956 phenolic resin. The silica fibers may have a diameter of about 1.5 um. The carbon fibers may be milled fibers having a length of about 150 um and a diameter of about 7.4 um. One specific carbon fiber that is suitable for use is Asbury Graphite Mills AGM-94 milled carbon fibers. The silica microspheres may have a diameter of between about 20-250 um; and the phenolic microspheres may have a most common diameter of between about 20-100 um. In one implementation Plenocet BJO-0930 phenolic microspheres are used. It will be appreciated, however, that all of these dimensions may be varied to suit the needs of a particular application.

The use of Plenco 11956 resin is particularly advantageous because it is a single component, water based resole phenolic resin that does not require adding flammable solvents, toxic curing agents, or reactive diluents to the basic phenolic resin, as with epoxy-novolac or some other types of phenolic resins. Because it is a liquid at room temperature it does not have to be heated to be blended with fillers. Because pure phenolic resin is a better ablator than typical curing agents or reactive diluents, the absence of such curing agents and reactive diluents from the phenolic resin helps to provide the ablative material 16 with superior thermodynamic response characteristics. The lack of a curing agent also allows the freshly mixed ablative material 16 to have a longer room temperature working life, since rapid cure does not initiate until it is heated to above 15° F. The fresh resin has a relatively long storage life at 0° F. of typically about four months and the room temperature working life for the ablator ablative material 16 is five days, unlike epoxy-novolac, or some other phenolic resin based ablators. These characteristics of Plenco 11956 resin, along with the use of frozen preforms, give the ablative material 16 the working time needed to apply it to large monolithic structures that can be cured in one piece. Some other phenolic resins or epoxy-novolac material systems either have short working lives that limit the area of the heat shield that can be processed at one time, or they require that the ablative material be hand injected into individual honeycomb cells using heated caulking guns.

Referring to flowchart 50 of FIG. 5, in forming the ablative material 16, it is preferred, but not absolutely essential, that the phenolic microspheres are dried out using a heated inert atmosphere before they are used to make the ablative material 16, as indicated at operation 52. This serves to remove any water and volatiles that may remain in the microspheres from the manufacturing process used to make them, and it stabilizes the state of cure of the phenolic resin that comprises the microspheres. The silica microspheres may also be dried by the same process to remove adsorbed water. It is also preferred that the various constituent materials used to form the ablative material 16 are added in a specific order to avoid clumping, achieve even wetting of the fibers and the microspheres by the phenolic resin, and to obtain uniform blending of all ingredients. The mixing may be done in a commercial bread mixer that imparts high shear forces to the blend but does not chop or mill the fibers and microspheres. To this end, at operation 54 a quantity of phenolic resin is provided, which as explained above is preferably Plenco 11956 phenolic resin. At operation 56 the silica fibers are added to the resin and mixed to achieve uniform dispersion and wetting by the resin. At operation 58 the carbon fibers are then added to the phenolic resin/silica fiber blend and the mixing is continued. At operation 60 the silica microspheres are added to the blend and the mixing is continued. At operation 62 the phenolic microspheres are added to the blend. At operation 64 the mixing is continued until the final uniform wetting and consistency are achieved. The sequence of adding ingredients and mixing is carried out over a time span of typically between about 23 minutes-30 minutes. In laboratory testing the ablative material 16 had a density (virgin) of 0.577 g/cm³ (36 lbm/ft³); a thermal conductivity (virgin) at room temperature of 0.107 W/m-° K. (0.62 Btu/hr-ft²); an ablation onset temperature, in Nitrogen, of 396° C. (744° F.); a tensile strength through its thickness of 4.08 MPa (592 lb/in²) and an effective heat of ablation of 69.9×10³ KJ/Kg (at a plasma arc jet heat flux of 420 W/cm²). At this point the ablative material 16 is ready to be manufactured into a preform.

Referring to FIGS. 6-10, one method for forming a preform comprised of the ablative material 16 will be described. In FIG. 6 the ablative material 16 is used to fill a mold 70. The mold will have dimensions of length, width and thickness that correspond to the desired dimensions for the preform. In FIG. 7 mold 70 is vacuum bagged with suitable bagging materials 72 and the ablative material 16 is debulked, (i.e., compressed to consolidate the granules of ablative material and remove voids by means of a partial vacuum drawn on the vacuum bag). In FIG. 8 the mold 70 with the ablative material 16 is frozen at approximately −10° F. for about 4-6 hours. In FIG. 9 the bagging material 72 is removed from the mold. The frozen ablative material preform 74 is shown in FIGS. 10A and 10B. The preforms can be used immediately for filling honeycomb core 14, or they can be freezer stored up to 2 months for later use. For a large heat shield 12, that requires a plurality of preforms, the necessary quantity of preforms are made in advance and freezer stored until the time of final assembly.

Referring now to the flowchart 100 of FIG. 11 and the drawing of FIG. 12, a description of using the preform 74 to form the heat shield 12 will be described. At operation 102 the plasma cleaned honeycomb core 14 is slotted on the carrier structure side (as shown in FIG. 4), using a diamond edged cutting tool, to thus form the slots 21. At operation 104 the carrier structure 20 is bonded to the honeycomb core 14. At operation 106 the honeycomb core 14 with the carrier structure 20 bonded thereto is placed in a mold tool 75 sized approximately to the dimension of the honeycomb core and its carrier structure. At operation 108 the frozen preform 74 is placed over an upper surface of the honeycomb core 14, that being the surface opposite to that which the carrier structure 20 is secured to. The entire assembly is covered with vacuum bagging materials 77 as indicated at operation 110. The assembly ready to be autoclave cured is shown in simplified form in FIG. 12. The core slots 21 that provide venting during filling and cure are shown in FIG. 12 at the intersection of the honeycomb core 14 and the carrier structure 20.

Referring further to FIG. 11, the assembly of FIG. 12 is then autoclaved cured to the “green state”, i.e. partially cured, as indicated at operation 112. Exemplary temperatures and pressures that may be used during the autoclave cure cycle are illustrated in the graph shown in FIG. 13. During the autoclave curing cycle the preform 74 is thawed and squeezed into the cells 15 a of the honeycomb core 14 under pressure until the cells 15 a are completely filled with the material of the preform 74, to thus form the heat shield 12. Rather than by autoclave processing, the preform 74 could also be compressed into the cells 15 a of the honeycomb core 14 by a mechanical press, as indicated by dashed lines 79.

Once the green state cure operation is complete, the heat shield 12 is removed from the mold tool 75 (or the mold tool disassembled), as indicated at operation 114. At operation 116 excess material from the preform 74 that extends above the honeycomb core 14 may be removed by machining or other means, and the edges, or periphery, of the heat shield may be beveled to reduce the effect of shrinkage stresses during the subsequent postcure operation. At operation 118 the heat shield 12 is returned to the autoclave for postcure to the final cure state of the ablative material 16. Exemplary temperatures and pressures that may be used during the autoclave postcure cycle are illustrated in the graph shown in FIG. 14.

At operation 120 non-destructive examination of the heat shield 12 by x-ray and ultrasonic methods may be performed to: 1) verify the integrity of the adhesive bonds between the cured ablative material 16, the honeycomb core 14, and the carrier structure 20; 2) verify that the honeycomb cells are all filled completely, top to bottom, (i.e. the cell fill is free of voids); and 3) verify that there are no internal cracks in the cured ablative material 16. At operation 122 the final outer mold line (OML) contour of the heat shield may be machined, if needed, to provide a particular, desired contour. At operation 124, edge closeouts 125, one of which is shown in FIG. 15, that have been manufactured in the same manner as the heat shield 12 described above, may be secured such as by an adhesive to the carrier structure 20 to close off the exposed peripheral edges of the heat shield 12. The edge closeouts 125 may also be adhesively bonded to the edges of the main portion of the heat shield 12, or they may be bonded only to the carrier structure 20 and the gaps between the closeouts 125 and the main portion of the heat shield 12 subsequently filled with a room temperature curing silicone elastomer. Also at operation 124 the edge closeouts 125 of the heat shield 12 are all non-destructively inspected.

Referring to FIG. 16, when making a monolithic non-planar heat shield for the spacecraft 10 shown in FIG. 1, a slotted honeycomb core 150, similar or identical in construction to honeycomb core 14, may be formed with the desired moldline needed to enable attachment of a finished heat shield to the outer surface of the spacecraft. In this regard a plurality of ablative preforms 74 may be cut to desired shapes and layed onto the honeycomb core 150, which has been secured with an adhesive layer 18 to the carrier structure 20, and then the entire assembly vacuum bagged and cured in an autoclave as a single piece assembly. Each of the preforms 74 may have chamfered edges 74 a to help interlock with adjacently placed preforms. An alternative for making a monolithic, non-planar heat shield as shown in FIG. 16 is to temporarily secure the slotted honeycomb core 14 or 150 to a male tool that matches the outer mold line contour of the spacecraft structure, hen layup, vacuum bag, press into the core, and autoclave cure the ablative material 16 of the frozen preform 74 in the manner described above. Then the monolithic ablator is removed from the tool, the inner mold line contour is verified or machined, and the single piece is secured by an adhesive to the spacecraft structure outer mold line.

In one variation of producing the ablative material 16, the material 16 may be forced through a mesh screen, for example a 100 mesh screen, (meaning a stainless steel wire screen with 100 openings per inch that are about 0.005 inches on a side), to form a pelletized ablative material. The pelletized ablative material may then be distributed over the cells of the honeycomb core 14 to completely fill the cells prior to vacuum bagging of the honeycomb core.

The methodology of the present disclosure thus provides a means for filling large areas of a honeycomb core structure at one time rather than filling each cell individually, or by filling tile size pieces of honeycomb core by machine or hand pressing material into both sides of the core, followed by curing and then machining to a finished shape for installation. This approach makes possible at least three major options for heat shield assembly. The first option is highly advantageous and involves pre-bonding the unfilled honeycomb core to the exterior of a spacecraft using an existing adhesive that has been certified for manned spaceflight. Thus, the present disclosure eliminates the need for the development and certification of a new attachment design for attaching the heat shield to a spacecraft using gore segments or tiles. The second advantageous option that the methodology discussed herein makes possible is that when a particular spacecraft design does not allow for processing the heat shield on the spacecraft, a single piece monolithic ablator assembly may be made on the side and then secured to the spacecraft by an adhesive in one operation. The third option, which has advantages for some spacecraft as well as for hypersonic aircraft, ground vehicles and stationary applications, is to make large preformed cured billets that are subsequently machined into panels, gores or large and small tiles. Options 1 and 2 are illustrated and compared in FIGS. 17A-17E and 18A-18F, respectively. In FIG. 17A the honeycomb core 14 is first bonded to the carrier structure 20 a that will ultimately form a portion of a spacecraft. In FIG. 17B the assembled honeycomb core 14 and carrier structure 20 are then cured in an autoclave 160. The honeycomb core 14 may then be filled with the ablative material 16 and then cured in the autoclave 160 (FIG. 17C). The outer mold line (OML) of the resulting cured assembly of FIG. 17C may then be machined to the desired shape and/or contour, as indicated in FIG. 17D. The resulting product is shown in FIG. 17E. In FIGS. 18A-18F, option two described above is illustrated. The ablative material 16 is first compressed into the honeycomb core 14. In FIG. 18B, the assembly shown in FIG. 18A is then autoclave cured in the autoclave 160 to form assembly 170. In FIG. 18C a machine tool 180 is used to machine the inner mold line (IML) of the cured assembly 170 to form machined assembly 185, which is shown in FIG. 18D. In FIG. 18D the machined assembly 185 is then bonded to the carrier structure 20 to form assembly 190. In FIG. 18E the outer mold line (OML) of the assembly 190 is machined with a machine tool 200. The finished product 205 is shown in FIG. 18F.

The heat shield 12 manufactured as described above is lighter than existing heat shields made from pre-existing approaches because of the greater mass efficiency of the ablator composition. The heat shield 12 also uses safe, non-toxic materials. The heat shield 12 allows two options for a monolithic heat shield design to be constructed that can be made in accordance with less complex manufacturing procedures traditionally employed in the manufacture of such heat shields. These benefits also help to reduce the cost of the heat shield 12 as well as the time needed to manufacture it. In particular, curing the ablative material 16 after it has been attached to the honeycomb/carrier structure avoids the need to form or machine a cured ablative material to match the contour of a heat shield carrier structure, which must take into account machining errors and any variations in each specific carrier structure piece.

Referring to FIG. 19, a system and method for providing a strain compliant coating to the heat shield 12 will be described. In some instances the curing of the frozen ablative preform 74 (FIGS. 10A and 10B) in an autoclave may cause a small or moderate degree of shrinkage of the ablative material 16. Since the honeycomb core 14 may be bonded to the carrier structure 20 before the frozen preform 74 is compressed into the cells 15 a of the honeycomb core, and since the carrier structure 20 will typically be comprised of one or more metal layers of material, when curing occurs the differences in the thermal coefficients of expansion between the ablative material 16 and the carrier structure 20 may occasionally introduce stress cracks into the cured ablative material 16. These stress cracks would need to be repaired before the manufacture of the heat shield 12 can be considered complete.

A highly advantageous method for substantially reducing, or entirely eliminating, the risk of stress cracks developing in the ablative material 16 is to coat the surface of the honeycomb core 14 with a strain compliant material before the frozen fiber preform 74 is compressed into the cells 15 a. This may be accomplished by dipping the honeycomb core 14, as shown in FIG. 19, into a reservoir 300 that is filled with a strain compliant material 302 so that the surfaces of the wall portions 15 are completely covered with the strain compliant material. In one embodiment the strain compliant material may comprise a two-part, chemically cured, silicone plastic material. In one specific embodiment RTV-560 silicone rubber compound is used. In another implementation the strain compliant material 302 may comprise a powdered rubber mixed with a matrix or resin as the binder. The resin or matrix could be the same resin or matrix that is to form the ablative material 16. In another implementation the strain compliant material 302 may comprise cork in powder form mixed with a binder such as resin or matrix. In still another implementation the strain compliant material may comprise a thermoplastic polymer.

The strain compliant material 302 will typically be in a viscous form that enables it to flow easily over all of the surfaces of the honeycomb core 14 while the honeycomb core 14 is submerged therein within the reservoir 300. It may also be possible to spray the honeycomb core 14 with the strain compliant material 302 via a spray gun provided the consistency of the precise formulation of the strain compliant material used 302 permits such a manner of application. One particular strain compliant material that would be suitable for application via a spray gun is nylon. Still further, the strain compliant material could be applied via a brush. However, it is anticipated that applying the strain compliant material 302 by dipping the entire honeycomb core 14 into a bath of the material 302 is likely to be the most expeditious way of coating the honeycomb core 14.

The strain compliant material 302 may also have added to it a powder, for example silica microballoons or high purity silica powder, that aids the ablative material 16 of the frozen preform 74 in adhering to the wall portions 15 of the honeycomb core 14 during the cure process. The powder may help to provide a more slightly textured surface that enables the ablative material 16 to better “grip” onto the strain compliant material. Alternative types of powders could comprise zirconia or alumina powder.

The strain compliant material 302 effectively increases the strain compliance of the heat shield 12 without the need to reformulate the ablative material 16 itself. This increased strain compliance reduces internal stresses that result from shrinkage when the frozen preform 74 is being compressed into the cells 15 a of the honeycomb core 14 and cured through the application of heat in an autoclave. If the strain compliant material 302 is a thermoplastic, it will soften during the curing operation when heat is being applied to the honeycomb core 14. The thermoplastic will then “flow” or deform while the ablative material 16 undergoes shrinkage. After the shrinkage has ceased, the honeycomb core 14 with the ablative material 16 filling its cells 15 a may be cooled, during which time the thermoplastic (i.e., forming the strain compliant material 302) will again harden. The strain compliant material 302, once re-hardened during the cool down period (i.e., following the cure operation) will contribute additional strength and stiffness to the completed heat shield 12.

The use of the strain compliant material 12 provides significant manufacturing benefits. For one, the ablative material 16 does not need to be reformulated in an attempt to control shrinkage during the cure process. Another benefit is that the ablative material 16 can be fully cured in a single operation, rather than using a less than mature cure cycle. Still another benefit is that there is no need to make relief cuts in the ablative material 16 to try and control the formation of stress cracks during the cure cycle. Still another benefit is that the strain compliant material 302 provides increased strain compliance that enables the heat shield 12 to better accommodate in-flight stresses resulting from structural deflections or deformations that the heat shield is subjected to.

While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art. 

1. A method for forming a heat shield, comprising: providing a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; applying a strain compliant material to the wall portions prior to filling of the cells with an ablative material; positioning an ablative preform, formed by an ablative material in the shape of a sheet, over an outer edge surface of the honeycomb core; forcing the ablative preform into the cells of the honeycomb core so that portions of the ablative preform material separate from one another and at least substantially fill the cells of the honeycomb core.
 2. The method of claim 1, further comprising curing the ablative material.
 3. The method of claim 2, wherein said curing of the ablative material comprises the operation of placing the honeycomb core with the ablative material in an autoclave for a predetermined cure time.
 4. The method of claim 3, further comprising fixedly securing a carrier structure having a metal layer to a surface of the honeycomb core prior to filling the cells of the honeycomb core with said ablative material.
 5. The method of claim 1, wherein the strain compliant material comprises a viscous, material that is flowed onto the honeycomb core.
 6. The method of claim 1, wherein the strain compliant material comprises a flowable material contained in a reservoir, and wherein the honeycomb core is dipped into the reservoir to coat the wall portions of the honeycomb core prior to filling of the cells with the ablative material.
 7. The method of claim 1, wherein the operation of filling the cells of the honeycomb core with an ablative material comprises filling the cells with an ablative material that includes a plurality of phenolic microspheres mixed with a phenolic resin.
 8. The method of claim 1, further comprising: prior to forcing the ablative preform into the honeycomb, forming the honeycomb core with slots along an inner edge surface that is opposite to the outer edge surface into which the ablative preform is to be forced, to form passages through which air trapped inside each of the cells of the honeycomb core is able to escape when the ablative preform is forced in the cells of the honeycomb core.
 9. A method for forming a heat shield, comprising: providing a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; flowing a viscous strain compliant material over the intersecting wall portions prior to filling of the cells with a phenolic ablative material; arranging an ablative preform in the form of an ablative preform sheet on an outer edge surface of the honeycomb core; forcibly urging the ablative preform sheet into the honeycomb core such that portions of the ablative preform sheet are separated from one another to substantially fill a plurality of cells of the honeycomb core over which the ablative preform sheet lies; and curing the honeycomb core having said cells at least substantially filled with said phenolic ablative material in an autoclave.
 10. The method of claim 9, wherein flowing a viscous strain compliant material over the intersecting wall portions comprises dipping said honeycomb core in a reservoir filled with said viscous strain compliant material.
 11. The method of claim 10, wherein dipping said honeycomb core in a reservoir filled with said viscous strain compliant material comprises dipping said honeycomb core in a reservoir filled with a thermoplastic polymer.
 12. The method of claim 9, further comprising: prior to forcing the ablative preform sheet into the honeycomb, forming the honeycomb core with slots along an inner edge surface that is opposite to the outer edge surface into which the ablative preform sheet is to be forced, to form passages through which air trapped inside each of the cells of the honeycomb core is able to escape when the ablative preform sheet is forced in the cells of the honeycomb core.
 13. A method for forming a heat shield, comprising: providing a honeycomb core having a plurality of intersecting wall portions forming a plurality of cells; providing a frozen ablative preform formed from a phenolic ablative material that is able to be handled as a preform sheet; flowing a viscous strain compliant material over the intersecting wall portions of the honeycomb core prior to filling of the cells with the phenolic ablative material of the frozen ablative preform; positioning the frozen ablative preform over a surface of the honeycomb core; using a pressure or a vacuum to force portions of the frozen ablative preform into at least a subplurality of the cells of the honeycomb core such that portions of the frozen ablative preform substantially fill the subplurality of the cells of the honeycomb core over which the ablative preform lies; and curing the honeycomb core in an autoclave.
 14. The method of claim 13, further comprising, prior to forcing the frozen ablative preform into the honeycomb, forming the honeycomb core with slots along an inner edge surface that is opposite to the surface into which the frozen ablative preform is to be forced, to form passages through which air trapped inside each of the cells of the honeycomb core is able to escape when the frozen ablative preform is forced in the cells of the honeycomb core. 